(a) Field of Endeavour
The device described herein relates generally to the field of fittings, couplings, and joints for thermoplastic pipes, and especially to prefabricated pipe fittings for field installation on such pipes.
(b) Description of Related Art
In industrial, commercial and residential settings, hard mechanical piping systems are used to transport liquids or gasses throughout the structure and surrounding grounds. These hard piping systems come in four basic types, depending on their respective materials: (i) metal, (ii) thermoset, such as fiberglass reinforced plastic (“FRP”), (iii) thermoplastic, and (iv) composites of these three materials. Composites further consist of thermoplastic liners reinforced with either metal or FRP structures. A new tri-layered composite piping system has been developed in recent years. This pipe consists of two thermoplastic pipes with a FRP reinforcement layer sandwiched between them.
The weakest part of any mechanical piping system is the fitting joint, which is almost always weaker than the pipe itself. These weaknesses are inherent in the various types of joining methods used. All the various piping systems are joined mechanically by flanging and/or threading. Metal pipes and fittings are also welded. FRP pipes and fittings use adhesives. Thermoplastic pipes and fittings use either thermal heat welding or solvent cementing.
The thermoplastic pipe industry has used thermal heat welding, also called thermal fusion, and solvent cementing as the two primary joining methods. Between thermal heat welding and solvent cementing, thermal heat welding is the most integrated joining method. Solvent cementing is still used extensively for polyvinylchloride (“PVC”) and chlorinated polyvinylchloride (“CPVC”) material pipe systems because a reliable thermal heat welded joining system has not been completely developed by the industry for these systems. The list of thermoplastic materials that can be and are joined by thermal heat welding, includes, without limitation: PVC, CPVC, polyethylene (“PE”), polypropylene (“PP”), polyvinylidene fluoride (“PVDF”), ethylene chloro tri fluoro ethylene (“E-CTFE”), perfluoroalkoxy alkane (“PFA”), and others.
In thermal heat welding, two thermoplastic surfaces are heated to a molten state and fused together under pressure. This process results in a homogeneous bond between the two surfaces. The three main joining methods for thermal heat welding are “butt” fusion, “socket” fusion and “electro” fusion. The end result of these three methods is a fusion weld on only one surface of the pipe.
Butt fusion welding heats the end of the pipes and fittings and brings them together in a melt state, or molten state, under pressure. In butt fusion welding, the ends of the pipes and fittings are the same dimensions. Butt fusion welding is widely accepted in the industry as the most integrated joining method, with the main drawback being head loss caused by the internal bead of thermoplastic material formed at the joint seam. Socket fusion welding heats the inside of the socket of the fitting and the outside of the pipe and inserts them into each other in a melt state under pressure. Socket fusion welding pushes the two melted surfaces together thus introducing stress at the joint area. It also has a crevice area where the interface between the pipe and the fitting is a sharp 90° angle. This 90° angle is shown in prior art references such as FIGS. 3A, 3C, and 4 of U.S. Pat. No. 6,293,311. This sharp 90° interface causes high stress under large bending moment forces, and this area is highly susceptible to crack initiation and propagation. Once a stress crack is started, the distance for the crack to propagate outside the fitting is minimal, and the joint will fail.
Solvent cementing of PVC and CPVC pipe employs a socket fitting like a socket fusion weld but with a taper in the fitting, which taper acts as a mechanical lock between the pipe and the fitting surfaces. The main weakness of solvent cementing is that the joint itself is mechanical because the two joining surfaces never do fuse together and thus may be separated with the application of a force. The corrosive nature of the media carried by the pipe may also chemically break down the cement, thereby weakening or breaking the joint.
Dual laminate pipe comprises a thermoplastic liner pipe that is reinforced with FRP on the outside for structural strength, thus yielding a two layered pipe. The thermoplastic liner is referred to as the corrosion barrier. Dual laminate piping is used where a corrosive atmosphere would prohibit the use of other pipes made of metal. Dual laminate pipes are among the most expensive piping systems that are used in industry. There are several methods for manufacturing and joining dual laminate pipes. Pipes and fittings are typically fabricated first, and then the outer FRP layer is removed around the joint area (typically by grinding it away) so that the thermoplastic liner pipe can be either thermally fused or solvent cemented to a fitting. Once the thermoplastic liner pipe has been joined to a fitting, FRP is then re-applied around the joint area for complete reinforcement.
In industrial settings, there is a common need to transport liquid media over long distances or across an industrial environment. Various pipes are used to meet these needs. For example, in the chemical processing industry, it is common to use steel pipe with corrosion barriers, or glass fiber reinforced pipe, or other types of dual laminate pipes. Similarly, it is common in the petroleum industry to use steel pipes in and around drilling equipment, transport and processing equipment, and in industrial distribution facilities. These types of pipe are used for their structural strength, their hoop strength that promotes the capacity to carry high-pressure media, and their resistance to the forces and stresses developed in the pipe when carrying high temperature and pressure media.
These conventional piping systems are expensive, labor intensive to build and maintain, and they have a relatively short lifecycle, sometimes lasting only five to ten years depending on the environment and the corrosiveness of the media. In many instances, these systems also require construction with heavy and expensive steel casings, components, and connection members. Saving weight with dual laminate pipe leads to a labor intensive and time consuming operation. Thermoset material is more expensive than thermoplastic, which greatly increases the cost of dual laminate piping systems compared to thermoplastic piping systems.
Consequently, it is advantageous to use thermoplastic pipes for their ease of use and for the cost effectiveness of designing, building, and maintaining thermoplastic piping systems. Thermoplastic piping systems are relatively easy to install. Equipment and methods for socket and butt thermal fusion have become standardized. However, past thermoplastic piping systems lack the structural strength to carry heavy media over long distances, and they perform poorly under high temperatures and pressures.
The tri-layered pipe described above has been recently used to address these problems. The main problem with these tri-layered thermoplastic pipes is that they are difficult to connect, and conventional thermoplastic pipe fittings do not adequately protect or seal with the unique tri-layered structure of these thermoplastic pipes. Likewise, conventional socket and butt thermal fusion connection methods are inadequate because they do not provide a seal that reliably protects the glass fiber reinforcing layer of the tri-layered, glass fiber reinforced, thermoplastic pipes (“TTP pipe”) pipe from the corrosive media often transported in industrial piping systems. Conventional thermal welds are also conducive to cracks propagating from 90° angles between the socket surface and the abutment surface of the fitting members.
Therefore, what is needed is an improved, prefabricated thermoplastic fitting configured to accommodate the unique structural features of a tri-layered thermoplastic pipe, and a method for using the same.